Diagnostic imaging systems which use multiple imaging modalities are known in the art. For example, it is known to combine CT and PET sub-systems in a multi-modal imaging system to obtain synergistic advantages above and beyond the imaging advantages provided by each type of imagining modality individually. With such a multi-modal system, one imaging modality provides one type of information—for example, the CT modality can provide structural or anatomical patient data—while the other imaging modality provides another type of information—for example, the PET modality provides functional (e.g., blood flow) patient data. Additionally, data from an initial or preliminary CT scan of a patient can be used to generate an attenuation map, which is used to compensate for bodily attenuation of the emissions registered by the PET imaging system.
In order to optimize the benefit of such dual imaging modalities, the images from the two different modes are fused together so that the treating physician can see more clearly and accurately the relationship between the different types of information (i.e., structural and functional). Because the different imaging systems are mounted on different support structures (gantries) and are located at different positions in space with respect to each other, it is necessary to generate a transformation function which brines the separate images into registration with each other and compensates for the difference in vantage points from which the different images are acquired. Such a transformation function is also required in order to relate attenuation data taken from one perspective to attenuation that would be experienced from another perspective. Generating such a transformation function requires that a calibration scan be conducted using a phantom with known geometries and geometric relationship to the patient bed and scanning apparatus.
One arrangement that has been used in the past for such calibration scans has been a pair of short rods installed on the patient bed, with the rods intersecting in three-dimensional space. While such an arrangement has worked well for calibrating a single-position scan, it has not worked so well for multiple-position scans, and errors of as much as 2.5 millimeters in image registration have been measured. It is believed that such errors arise because the precise motion of the patient bed is not perfectly known or predictable over the entire scan range, which results in misalignment between the bed, PET, and CT positions for anything more than a one- or two-position scan. Thus, such a two-rod phantom arrangement is not ideally suited for calibrating a multiple-position scan such as a whole-body scan, which may require as much as five to seven scan positions.